The friction angle is the slope of the curve representing on a graph the frictional force between two bodies in contact, parallel to the surface of contact, plotted on the vertical axis, versus the force normal to the surface of contact with which the bodies are pressed against each other, plotted on the horizontal axis. Mathematically, the friction angle .phi. is defined as follows: ##EQU1## where
Fc=frictional force between the two bodies in contact
Fn=force normal to the surface of contact
The friction angle of surfaces of rock materials is often used as one parameter to measure the degree of roughness and alteration of rock fracture surfaces.
Basic friction angles can be generally measured either in a laboratory or on site using a shear test apparatus. Such apparatus consists of two half portions forming a box, each portion being adapted to contain one part of a rock sample which is cut in half by means of a saw blade. Preparation of the test sample requires cutting of the rock sample in two parts, fixing each part in one half of the box, with the cut surface facing up, pouring grout mix around each half sample and waiting for the grout to set. It is thus time consuming. The test itself is performed as follows:
The upper half of the box is pressed down against the lower half with an hydraulic press thus placing the facing cut surfaces under a specific amount of normal load. The bottom part of the box is then moved back and forth horizontally with the help of a hydraulic jack and the shear force required to move the bottom part of the box is measured. The test is performed with different values of normal load.
If the saw cut surface is not perfectly planar, the lateral movement of the bottom part of the sample will shear off micro-roughnesses on the contacting surfaces of the sample and produce crushed material. The friction angle obtained during such test approximates the residual friction angle. When the rock surface is perfectly planar with no roughnesses due to geometric asperities, the friction angle is characteristic of the material itself and is called the basic friction angle. The residual friction angle is slightly lower than the basic friction angle but in many cases, both values are interchangeable. Such values are significantly lower than the so-called peaks friction angle which takes into account the effect of the roughness due to the geometry of the surface of contact between the two parts of a natural rock fracture.